In recent years, in order to effectively utilize tight RF (Radio Frequency) band, a cognitive wireless communication technology is attracting attentions. IEEE (The Institute of Electrical and Electronics Engineers, Inc.) 802.22, which is a typical cognitive wireless communication standard, specifies to secondary use frequency band assigned for television broadcast which is from 54 MHz to 862 MHz. At the same time, communication devices conform to a plurality of standards among major wireless communication standards covering frequency band of VHF (Very High Frequency) to UHF (Ultra High Frequency) are developing extensively. In this context, it is highly desired to realize a transmitting and receiving apparatus which can be used for a wide range of RF band.
FIG. 17 is a figure showing a structure of the transmitting and receiving apparatus in relation to the present invention.
First, a receiving system in a transmitting and receiving apparatus 750 shown in FIG. 17 will be described. A RF signal is inputted through an antenna 8000 to a RF part which includes a band selection filter 8001, a LNA (Low Noise Amplifier) 8002 and a frequency converter 8003. In order not to saturate circuits at the following stages, the band selection filter 8001 removes signals included in undesired frequency bands from the received RF signal. However, the band selection filter 8001 cannot remove disturbing signals having frequencies relatively close to the desired signal. A carrier signal which passes the band selection filter 8001 is amplified by the LNA 8002, and the frequency converter 8003 converts to BB (Base Band) frequency. Then, the signal whose frequency is converted is performed additional signal processing such as filtering and amplitude adjustment by a BB filter 8004 and a VGA (Variable Gain Amplifier) 8005. The signal which the signal processing was performed is outputted as the RXBB (Receiving BB) signal.
Then, a transmitting system will be described. As is similar to the receiving side, after amplitude adjustment and a filtering process are performed by a VGA 8006 and a BB filter 8007, a TXBB (Transmitting BB) signal which should be transmitted is converted to a carrier frequency by a frequency converter 8008. Then, the signal which is performed the frequency conversion is amplified by a PA (Power Amplifier) 8009, and then suppresses undesired waves by the band selection filter 8001 and radiates from the antenna 8000.
A role of a band selection filter at the transmitting side includes suppression of undesired signals which is converted of its frequency by odd order higher of harmonics of a LO (Local Oscillation) signal. In addition, as a role of the band selection filter at the receiving side, it is mentioned that it suppresses disturbing signals which exist close to a frequency of odd order of the local oscillation signal. When a typical multiplier, which is represented by a double balanced mixer, is used for the frequency converter 8008, the band selection filter 8001 is required to remove the undesired waves caused by the third harmonics which is included in the local oscillation signal. Accordingly, a ratio of an upper bound frequency to a lower bound frequency of the passband of the band selection filter 8001 should be less than three times.
Consequently, for the transmitting and receiving apparatus which can cover wide frequency bandwidth, it should appropriately use a plurality of band selection filters. For example, in order to cover a frequency band from 50 MHz to 900 MHz, total three kinds of band selection filters including passband respectively from 50 MHz to 150 MHz (=50 MHz×3), from 150 MHz to 450 MHz (=50 MHz×3) and from 450 MHz to 900 MHz are required.
The transmitting and receiving apparatus, which conforms to wireless communication standards having a relatively wide frequency bandwidth and conforms to a plurality of wireless communication standards including the cognitive wireless communication, is required to reduce a number of components and also integrate circuits as much as possible, in order to reduce a cost and minimize a mounting area. In addition, the band selection filter should have performance characteristics including low noise, low loss, high linearity and high breakdown voltage.
These performance characteristics of the band selection filter heavily depend on performance characteristics of passive elements such as surface acoustic wave filters and LC filters. However, it is difficult to integrate high-performance passive elements on a chip top, and even if it can integrate the passive elements on it, the cost becomes too much increased. In addition, as it has described above, a number of the band selection filters will increase for the transmitting and receiving apparatus which conforms to wireless communication standards which specify wide relative frequency bandwidth or covers a plurality of wireless communications.
In order to reduce the number of the band selection filter, a harmonics suppression mixer which adapts a harmonics suppression function in the frequency converter has been used. FIG. 18 is a figure showing waveforms of the local oscillation signal at the harmonics suppression mixer related to the present invention. FIG. 18 shows waveforms of a local oscillation signal having three phases where the phases are different in 45 degrees respectively, and an effective local oscillation signal composed from those.
The harmonics suppression mixer multiplies the three phases of the local oscillation signal whose phases are different in 45 degrees respectively by an input signal, and performs weighted addition for each amplitude with ratio of 1:sqr(2):1. Where, sqr(2) means a square root of 2. Consequently, it can effectively suppress the third harmonics component and the fifth harmonics component of the local oscillation signal.
Accordingly, when it uses the above-mentioned harmonics suppression mixer, the band selection filter should be able to suppress no smaller than the seventh harmonics. That is, it should set the ratio of the upper bound frequency to the lower bound frequency in the passband of the band selection filter to less than 7 times. Consequently, it can reduce the number of the band selection filter by using the harmonics suppression mixer. In the above-mentioned example, two kinds of filters consisting of from 50 MHz to 350 MHz (50 MHz×7) and from 350 MHz to 900 MHz are sufficient for the required band selection filters. Where, the harmonics suppression mixer multiplies the receiving signal or the transmitting signal by the local oscillation signal with three phases. Consequently, three multipliers are required and a size of circuit increases to about triple accordingly. In addition, In order to get the local oscillation signals whose phases are different in 45 degrees respectively, normally, a signal having quadruple frequency to the local oscillation frequency is required. Consequently, power consumption of the harmonics suppression mixer also becomes large.
On the other hand, under a condition that triple of the local oscillation frequency will be outside of the passband of the band selection filter, influences by the third harmonics of the local oscillation signal is sufficiently suppressed by the band selection filter. For example, when it uses the band selection filter having frequency band from 50 MHz to 350 MHz and the local oscillation frequency is no smaller than 350 MHz/3 (=about 117 MHz), the frequency of the third harmonics will be no smaller than 350 MHz. In this case, the harmonics suppression of no smaller than third harmonics is not required for the frequency converter.
However, in this kind of case that the harmonics suppression by the frequency converter is not required, when it uses the harmonics suppression mixer, it generates extra power consumption which is not fundamentally required to the harmonics suppression mixer.
FIG. 19 is a figure disclosed in the non-patent literature 1 showing a structure of a frequency converter in relation to the present invention. A frequency converter 950 shown in FIG. 19 collocates a simple double balanced mixer 9000 and a harmonics suppression mixer 9007, and selects a mixer to be used according to a carrier frequency. The frequency converter 950 suppresses increment of power consumption by using the double balanced mixer 9000 instead of the harmonics suppression mixer 9007, when the harmonics suppression is not required.
The harmonics suppression mixer 9007 includes three multipliers 9001, 9002 and 9003 and an adder 9004. In addition, a half frequency demultiplier 9005 is a circuit for generating the local oscillation signals whose phases are different in 90 degrees respectively which are required for orthogonal modulation and demodulation, and driving a double balanced mixer 9000. Where, FIG. 19 shows a signal path of only one system among two systems which are in the orthogonal position. In addition, the one-fourth frequency demultiplier 9006 is a circuit for generating signals whose phases are different in 45 degrees respectively for the harmonics suppression operation, and driving three multipliers 9001, 9002 and 9003.
In relation to the above mentioned technology, the patent literature 1 discloses a structure of a mixer in which it multiplies three local oscillation signals whose phases are different in 45 degrees respectively by an input signal and adds them after the amplitude is amplified with ratio1:sqr(2):1.
In addition, the patent literature 2 also discloses a structure of a mixer in which it multiplies three local oscillation signals whose phases are different in 45 degrees respectively by an input signal and then adds them.